Circuit for converting AC voltage into DC voltage

ABSTRACT

The invention relates to a circuit for converting AC voltage into DC voltage for one or two consumers (Rm, Ra), in which circuit a first part of the current is fed to the input side via a large induction coil (L 50 ) and a first rectifier (Gm) and a second part of the current via a second rectifier (Ga) upstream of the large induction coil. On the output side, behind the second rectifier (Ga), a switched-mode power supply is arranged, preferably in the form of an up-converter or a flyback converter ( 10 ). The division of the applied current for smoothing the output voltage provides that the induction coil (L 50 ) can be accordingly dimensioned smaller and, despite this, the circuit can also satisfy relevant standard specifications.

[0001] The invention relates to a circuit for converting AC voltage intoDC voltage for at least one consumer, comprising

[0002] two external terminals for feeding an AC voltage,

[0003] a rectifier module whose two inputs are connected via connectionlines to the two external terminals and which comprises two output lineswith external terminals for coupling this at least one consumer,

[0004] at least one induction coil coupled in at least one of theconnection lines between the external terminals and the rectifiermodule.

[0005] Many electric or electronic devices need a DC voltage for theiroperation, which DC voltage is to be rendered available or generated,respectively, from the AC voltage of the electric mains. A televisionset (TV) is mentioned in place of such terminal devices. The circuitsfor converting AC voltage into DC voltage installed in such devices areto satisfy certain requirements with regard to their conrversionbehavior, particularly, with regard to the remaining portion of thenetwork harmonics. Such requirements are known, more particularly, fromthe subject matter of standards, for which especially CEI/IEC 61000-3-2,SECOND EDITION, 2000-08 may be mentioned, which is to be satisfied inEurope as from the year 2001 by terminal devices that have a powerconsumption exceeding 75 W.

[0006] To guarantee that said quality standards are adhered to, thereare various possibilities with various optimal power ranges. Activesolutions are predominant for large powers of about 500 W and upwards.In range of smaller powers, on the other hand, often 50 Hz coils arefound. They are comparatively inexpensive, but also comparatively largeand heavy. This size or this concentrated volume respectively leads toproblems with the available place on printed circuit boards or toproblems with chock resistance of the manufactured devices.

[0007] Said problems particularly occur in applications which containmore than one switched-mode power supply. An example of this is formedby modern TV sets in which the second power supply is necessary forstandby operation and in normal operation supplies power to the logicICs.

[0008] Against this background it was an object of the present inventionto provide a circuit for converting AC voltage into DC voltage for atleast one consumer, which circuit guarantees sufficient quality of theconversion with limited required space and expenditure and which isparticularly suitable for applications having two switched-mode powersupplies.

[0009] This object is achieved by a circuit having the characteristicfeatures of claim 1. Advantageous embodiments are stated in thedependent Claims.

[0010] The circuit for converting AC voltage into DC voltage for atleast one consumer accordingly comprises the following component parts:

[0011] (a) Two external terminals (EA) to which the poles of an externalAC voltage source can be connected.

[0012] (b) A rectifier module of known structure whose two inputs areconnected to one of said external terminals via connecting lines andwhich has two output lines to which at least one consumer can beconnected.

[0013] (c) At least one induction coil, which is arranged in one of theconnecting lines which connect the external terminals to the rectifiermodule. The induction coil has for its object to level the rechargingcurrent or reduce the harmonic part of the recharging current,respectively.

[0014] The circuit is characterized in that at least a second rectifiermodule is present whose two inputs are connected to a respectiveexternal terminal without an induction coil being arranged in theassociated connecting line. Furthermore, the rectifier module has twooutput lines for connecting at least one consumer. A switched-mode powersupply with a respective drive is then arranged for this purpose in theoutput lines, which power supply has at least a substantially sinusoidalinput current. This means that the input current has a small harmonicpart. Such power supplies which are also called AC/DC converter or PFC(Power Factor Correction circuit) are known from the literature (compareSiemens “Schaltnetzteile” W. Hirschmann, A. Hauenstein, 1990, ISBN3-8009-1550-2, chapters 6.1 to 6.3).

[0015] In the circuit according to the invention, the power suppliedfrom the exterior as AC voltage is led over two separate inputs paths.One path leads via the first rectifier module. This path includes inknown manner an induction coil to effect a reduction of the harmonicpart of the recharging current. In addition, according to the invention,a second input path is opened via the second rectifier module. Theconsequent relief of the first input path when the total powerconsumption remains the same may be used for accordingly arrangingsmaller induction coils in this path, because the necessary inductivitycan be chosen smaller. In this way it becomes possible to use inductioncoils of drastically reduced size which can also be used when there islimited space on printed circuit boards and to avoid problems with theshock resistance of the devices. Furthermore, the pattern of the powerconsumption from the AC voltage source, which pattern is caused by thecircuit according to the invention, is modified in advantageous manner,so that standard specifications can be satisfied in simple fashion atreduced cost.

[0016] Whereas, basically, different topologies can be used for theswitched-mode power supply, this power supply is preferably arranged sothat it can be operated with a sinusoidal current. For this purpose,preferably an upconverter (also called boost converter or forward gaincontroller) and/or a flyback converter, for contrary to many othertopologies, the upconverter and the flyback converter can generate apurely sinusoidal input current when accordingly driven in a “normal”fashion.

[0017] As an alternative, the switched-mode power supply (for exampleupconverter or flyback converter) can also be operated not to generate apurely sinusoidal input current, but use the recharging pulse to keepthe input current tapped from the external terminals sinusoidal as longas possible.

[0018] The circuit according to the invention may be complemented invarious ways to obtain improved output signals. For example, the twooutput lines of the first rectifier module Gm may be coupled each via asmoothing capacitor. This smoothing capacitor provides a suppression ofhigher frequencies in the output voltage.

[0019] The coupling via the output capacitor of the switched-mode powersupply (for example, of the upconverter or flyback converter) takesplace in the second output path.

[0020] These and other aspects of the invention are apparent from andwill be elucidated, by way of non-limitative example, with reference tothe embodiment(s) described hereinafter:

[0021] In the drawings:

[0022]FIG. 1 shows a circuit according to the state of the art forsupplying power to two consumers;

[0023]FIG. 2 shows a circuit according to the state of the art forsupplying power to one consumer;

[0024]FIG. 3, shows the curve of the power consumption in a circuitshown in FIG. 1 or 2,

[0025]FIG. 4 shows a circuit according to the invention with twoconsumers;

[0026]FIG. 5 shows the curve of the power consumption in the circuitshown in FIG. 4 when the AC/DC converter is operated with sinusoidalcurrent; and

[0027]FIG. 6 shows the curve of the power consumption of the circuitshown in FIG. 4 when the AC/DC converter is operated when part of therecharging current pulse is used.

[0028] The generation of DC voltages for supplying power to electronicterminal devices from an AC mains current is to satisfy certain minimumrequirements with regard to the conversion of the AC voltage into DCvoltage. For consumers consuming more than 75 W these requirements areexpressed, for example, in the intended standard CEI/IEC 61000-3-2,SECOND EDITION, 2000-08. Various methods of satisfying this standard areabout generating as exact a sinusoidal input current as possible.Nevertheless, such a sinusoidal form is not absolutely prescribed by thestandard.

[0029] A widely used method of satisfying quality standards consists ofa passive solution via a so-called 50 Hz coil which smoothes the inputcurrent. However, a high inductivity is then required to satisfy forexample the CEI/IEC 61000-3-2, SECOND EDITION, 2000-08. The respectivecoil is thus very large and heavy, which causes problems whenaccommodating it on a printed circuit board and also during shock testsof the devices. The present invention renders a circuit available whichmakes the use of smaller coils possible while simultaneously satisfyingthe quality specifications for the conversion.

[0030]FIG. 1 shows a circuit according to the state of the art forsupplying a DC voltage to two consumers Rm, Ra, which DC voltage is theresult of a conversion from an AC voltage source. This circuitcorresponds to that of FIG. 2 in which the two consumers Rm, Ra areconnected in parallel to the output circuit of the rectifier module G.

[0031]FIG. 2 shows a more detailed picture of a known circuit forsupplying a DC voltage to a consumer RL. The power is taken from an ACvoltage source and converted into an uncontrolled or fluctuating (rippleaffected) DC voltage by a rectifier module G. The AC voltage is fed tothe input terminals of the rectifier module G. One of the connectinglines or both connecting lines of the rectifier module G to the externalterminals EA of he circuit include a 50 Hz coil 50 for smoothingpurposes. The DC voltage can be tapped from the output terminals “+”,“−” of the rectifier module G by the consumer RL. The two output linesto these terminals will be coupled via a smoothing capacitor C.Furthermore, an output line often includes a resistor R to achieve alimitation of the switch-on current.

[0032] The curved pattern of the power consumption resulting from thecircuit shown in FIG. 1 or FIG. 2, respectively, is shown in FIG. 3where the circuit is based for example on the following parameters:R=1.5 Ohms, L50−=32 mH, C=22 μF, Uin=230 V, f=50 Hz, Po=145 W.

[0033]FIG. 4 shows a circuit according to the invention for supplyingpower to two consumers Rm, Ra. In this circuit there are two differentpaths through which energy is drawn from the AC voltage (mains powersupply) present on the external terminals EA. The first path leadsthrough the first amplifying module Gm (m=“main”), the second paththrough the second rectifier module Ga (a=“aux”). Only the first pathleads on the input side of the first rectifier module Gm through the 50Hz coil L50, while the auxiliary line for the second path is tappedbefore the 50 Hz coil L50.

[0034] While the output path via the rectifier module Gm utilizes theconventional configuration of diode bridge and electrolyte capacitor Cm,the power for the second path is actively rendered available by means ofthe sinusoidal current. This may be effected via a switched-mode powersupply 10 (also called AC/DC converter or PFC) such as, for example, anupconverter (boost converter) or a flyback converter, which is arrangedbetween the rectifier module Ga and the consumer Ra. The first pathwhich includes the rectifier module Gm then preferably renders thelarger power available. However, the first path could also process thesmaller power.

[0035]FIG. 5 shows the resulting signal curve of the power consumptionon the external terminals EA (50 Hz input current, a half-wave) when thecircuit according to FIG. 4 is used, if the power for the consumer Ra isrendered available via a sinusoidal current flowing through therectifier module Ga. The resulting form of the curve of the inputcurrent (solid line) consists of two parts, that is to say, the currentflowing through the first rectifier module Gm (dashed line) and thecurrent flowing through the second rectifier module Ga (dotted line).The main recharge pulse is limited by the large 50 Hz coil L50.

[0036]FIG. 6 shows the resulting signal curve of the power consumptionon the external terminals EA (50 Hz input current, a half-wave) when thecircuit shown in FIG. 4 is used, if the power for the consumer Ra is nottapped as a purely sinusoidal power, but part of the recharge pulse isused to form as sinusoidal an input current as possible. The resultingform of the curve of the input current (solid line) again consists oftwo parts, the current flowing through the first rectifier module Gm(dashed line) and the second rectifier module Ga (dotted line). When therecharge pulse begins, the current consumption is reduced by the secondrectifier module Ga until the power consumption becomes negative (whichis not possible because of the rectifier module Ga). For this purposethe recharge pulse is to be detected in the upper circuit. This may beeffected according to present day current sensor principles.

[0037] In the examples on which FIGS. 5 and 6 are based the coil L50 canbe reduced by a factor of 2 compared to the state of the art (withL×1{fraction (2/2)} as an approximate value for the size of the coil).

[0038] In all proposals the non-linear magnets may be used for furtherreduction of the size of the magnetic components. List of preferences:AC AC voltage source C smoothing capacitor Ca capacitor in the secondinput path Cm capacitor in the first input path EA external terminal Gmrectifier in the first input path (main) Ga rectifier in the secondinput path (aux) 1 current L50 50 Hz coil Lcom current inductivity Rohmic resistance RL consumer Rm, Ra consumer in the first/second inputpath t time

1. A circuit for converting AC voltage into DC voltage for at least oneconsumer, comprising, two external terminals (EA) for feeding an ACvoltage, a rectifier module (GM) whose two inputs are connected viaconnection lines to the two external terminals (EA) and which comprisestwo output lines with external terminals for coupling this at least oneconsumer (Rm), at least one induction coil (L 50) arranged in one of theconnection lines between the external terminals (EA) and the rectifiermodule (Gm), characterized in that at least a second rectifier module(Ga) is present whose two inputs are each connected to one of theexternal terminals (EA) while bypassing the induction coil (L50) andwhich rectifier module (Ga) has two output lines for the connection ofat least one consumer (Ra) while the output lines include aswitched-mode power supply (10) arranged between the second rectifiermodule (Ga) and the consumer to be connected.
 2. A circuit as claimed inclaim 1, characterized in that the switched-mode power supply (10) isarranged so that it is operated with a substantially sinusoidal current.3. A circuit as claimed in claim 1 or 2, characterized in that theswitched-mode power supply is an up-converter or a flyback converter(10).
 4. A circuit as claimed in at least one of the claims 1 to 3,characterized in that the switched-mode power supply (10) is arranged sothat it utilizes part of the recharge pulse to extract as sinusoidal acurrent as possible at the two external terminals (EA).